A pressure-fed engine, optimized for altitude operation. First Flown: August 1982, Delta 164. Mounting: fixed. Engine Cycle: pressure-fed. Oxidizer: nitrogen tetroxide at 9.1 kg/sec. Fuel: Aerozine-50 at 4.76 kg/sec. Mixture Ratio: 1.9:1. Thrust: 43.38 kN vacuum. Isp: 320.5 sec vacuum. Expansion Rate: 65:1. Combustion Chamber Pressure: 8.84 atm. Cooling Method: ablative chamber, radiative skirt. Burn Time: qualified up to 500 sec (unlimited starts). The engine produced approx. 212-213 seconds of Isp when tested with a 1:1 nozzle at sea level for acceptance testing. The engine is not regeneratively cooled. It uses an ablative made from rubber modified silica phenolic at the combustion flame front.